In electrohydraulic servo systems which embody a servo valve coupled to a hydraulic actuator, particularly a linear actuator, it is conventional practice to monitor actuator position using an electroacoustic linear displacement transducer for example as marketed by Temposonics Inc. of Plainview, N.Y. and disclosed in U.S. Pat. No. 3,898,555. As conventionally employed in such systems, this transducer includes a magnet coupled to the actuator piston for motion conjointly therewith, and an electroacoustic waveguide adjacent to the path of the magnet. A current pulse is launched on a wire which extends through the waveguide and coacts with the field of the magnet to propagate an acoustic signal within the waveguide. A coupler or mode converter receives such acoustic signal, with the time between launching of the current pulse and receipt of the acoustic signal being a function of position of the magnet relative to the waveguide. This transducer is durable, is directly mounted on the actuator cylinder but magnetically rather than physically coupled to the actuator piston, and is capable of providing an accurate indication of actuator piston position. However, conventional electronics for obtaining such position reading are overly complex and inordinately expensive. Furthermore, such electronics are conventionally supplied in a separate package which must be appropriately positioned and protected in the actuator operating environment.
Copending U.S. application (V-3985) Ser. No. 849,540, filed Apr. 8, 1986, now abandoned, and assigned to the assignee hereof, discloses an electrohydraulic servo valve assembly which includes a servo valve and microprocessor-based control electronics mounted in a single package for connection to hydraulic equipment, such as a linear actuator. In a particular implementation of such disclosure in a servo-valve/linear-actuator combination, improved circuitry is featured for monitoring operation of the Temposonics type electroacoustic transducer. An initial current pulse is launched in the waveguide in response to a measurement demand from the micro-processor-based control electronics, and a counter is simultaneously reset. Upon receipt of the acoustic return pulse from the waveguide, the counter is automatically incremented and a current pulse is relaunched in the waveguide. The output of the counter includes facility for preselecting a number of launch/return cycles in the waveguide, and for generating an interrupt signal to the microprocessor-based control electronics to indicate that the preselected number of recirculations has been reached. An actuator position reading is stored in a clock which measures the amount of time between the initial measurement demand signal and the interrupt signal. The clock output is transmitted to the control microprocessor on demand.
Although the combination of the Temposonics type transducer and monitoring electronics disclosed in such copending application is considerably less expensive than that previously proposed, and is reliable in long-term operation, improvements remain desirable. For example, electronics for obtaining a measurement reading in the disclosure of the copending application occupies one-third of the total electronics package. Reduction in the quantity of required circuitry is desirable to reduce power dissipation and increase space available for implementing other control features. Furthermore, although a measurement reading is obtained very quickly relative to motion of the actuator piston, the system of the copending application does not continuously monitor piston position in real time.
A general object of the present invention, therefore, is to provide apparatus for determining position of a movable member, particularly for determining position of the piston in an electrohydraulic linear actuator, which is inexpensive to implement as compared with like devices in the prior art, which reduces the overall quantity of circuitry necessary to monitor motion, which is adapted to continuously monitor motion in real time, which is accurate to a fine degree of resolution, and/or which is reliable over a substantial operating lifetime.